Recently, in devices such as portable telephones and portable computers, picture image information is incorporated in an image pickup device. In an image recording or photographic optical system used in such an image pickup device, it is necessary to make the optical system light-weight and compact in order to obtain the desired portability. When the optical axis of an objective lens extends in the thickness direction of an image pickup device, a technique for reducing the thickness of the casing of the device has been developed that uses a prism to bend the optical axis of the optical system.
On the other hand, additional functionality is also desirable in image recording and photographic optical systems used in such image pickup devices, and image recording and photographic optical systems that include a zoom function have been proposed, for example, in Japanese Laid-Open Patent Application 2004-264585 and Japanese Laid-Open Patent Application 2000-131610.
However, in Japanese Laid-Open Patent Application 2004-264585, in the case of forming the zoom lens of three lens groups With the third lens group from the object side being movable both for zooming and for focusing, the third lens group is moved continuously toward the object side during zooming toward the telephoto end so that the distance between the second lens group and the third lens group becomes small. Thus, little distance is left for assuring sufficient movement of the third lens group for focusing. Particularly, in short-distance photography, focusing movement becomes greater at the telephoto end than at the wide-angle end so that the focusing lens, which in this case is the third lens group, requires increased movement. That is, large movements are required at the telephoto end. Japanese Laid-Open Patent Application 2000-131610 operates differently with a zoom lens that includes four lens groups, but the use of the four lens groups limits miniaturization and reduction of the costs of the zoom lens.